Though the kinds of phenomena are not very numerous, all the other considerations are ways the quant can differentiate his approach from those of others who are ostensibly looking for the same types of anomalies. Depending on the time horizon of the strategy and number of positions it takes, the number of trades per year can easily get into the millions. I know many traders who execute 10,000 to 100,000 trades each trading day. As you can imagine, small differences in how one arrives at a single trade are amplified when millions of trades are made in a year.